In electro-acoustics there is a need for a system which simultaneously provides good quality high and low frequency bandwidths, whilst being relatively simple, robust, reliable and cheap. Maintaining the high frequency output of a compression driver requires a phase-plug closely spaced to the diaphragm to avoid an excessive acoustic compliance. The smaller this spacing the more extended the high frequency bandwidth will be. This phase plug to diaphragm spacing also limits the maximum diaphragm displacement since if the diaphragm contacts the phase plug gross distortion or even mechanical failure will occur. Since lower frequencies require larger volumes of air to be displaced, the smaller the phase plug spacing the less low frequency output is possible.
Consequently, increasing the low frequency output of a compression driver while simultaneously maintaining the high frequency bandwidth extension cannot be achieved by increasing the maximum diaphragm displacement.
In principle, increasing the size, and hence radiating area, of a diaphragm increases the low frequency output without reducing the high frequency bandwidth. However, practical diaphragms suffer non-pistonic vibrational modes at high frequencies which cause response irregularities and limit the usable high frequency bandwidth. Increasing the diaphragm size decreases the frequency of these modes thus limiting the diaphragm size possible for a particular material and geometry. Consequently, compression drivers of a similar size diaphragm and diaphragm material have similar limitations on acoustic output and bandwidth.
Conventional compression drivers with bandwidth extending to high frequencies fall into two main categories of diaphragm geometry. The diaphragm is either in the form of a spherical cap, or is an annular diaphragm which typically has a V-section, as in U.S. Pat. No. 6,804,370 and U.S. Pat. No. 5,878,148.
Annular diaphragms are usually only a centimeter or so wide and thus may be fabricated from lightweight material such as mylar film. The area is not as large as a spherical cap compression driver of the same diameter but extended high frequency response may readily be obtained.
Where extended bandwidth from one source is required, a coaxial configuration of two drivers is a somewhat complicated but viable option. In this configuration one large and one small diaphragm is driven through an electrical dividing network so the high frequencies are generated by the small diaphragm and the low frequencies by the large diaphragm. The output of the two diaphragms is combined using a complicated network of acoustic paths. Since the output of one diaphragm may travel down the entrance to the other diaphragm there are a number of additional acoustic resonances which may limit sound quality and bandwidth. The diaphragms also couple: the radiation from one causes the other to move. U.S. Pat. No. 5,878,148 teaches that the use of two annular diaphragms results in a compact design with relatively short acoustic channel lengths between the diaphragms and the manifold where the acoustical outputs are combined. However, even in this instance the acoustical interactions between the diaphragms are a significant limitation to the performance of coaxial drivers. None the less, this configuration is frequently preferred to a single large spherical cap type driver due to the poor sound quality resulting from structural resonances within the diaphragm, former and surround of the latter.